JOHNSON CONTROLS
159
SECTION 8 – UNIT OPERATION
FORM 150.72-ICOM7
ISSUE DATE: 10/22/2020
8
The return chilled liquid set point is programmable
from 40°F to 70°F (4.4°C to 21.1°C) in water chilling
mode and from 10°F to 70°F (-12.2°C to 21.1°C) in
glycol chilling mode. In both modes, the cooling range
can be from 4°F to 20°F (2.2° to 11.1°C).
As an example of compressor staging (See Table 25
on page 157), a chiller with six compressors using a
Cooling Set Point programmed for 45°F (7.20°C) and a
Range Set Point of 10°F (5.56°C). Using the formulas
in Table 25 on page 157, the control range will be
split up into six (seven including hot gas) segments,
with the Control Range determining the separation be-
tween segments. Note also that the Cooling Set Point is
the point at which all compressors are OFF, and Cool-
ing Set Point plus Range is the point all compressors
are ON. Specifically, if the return water temperature is
55°F (12.8°C), then all compressors will be ON, pro-
viding full capacity. At nominal gpm, this would pro-
vide approximately 45°F (7.2°C) leaving water tem-
perature out of the evaporator.
If the return water temperature drops to 53.4°F (11.9°C),
one compressor would cycle OFF leaving five com-
pressors running. The compressors would continue to
cycle OFF approximately every 1.7°F (.94°C), with the
exception of hot gas bypass. Notice that the hot gas
bypass would cycle ON when the return water tem-
perature dropped to 46.25°F (7.9°C). At this point one
compressor would be running with hot gas.
Should the return water temperature rise from this point
to 46.7°F (8.2°C), the hot gas bypass would shut OFF,
still leaving one compressor running. As the load in-
creased, the compressors would stage ON every 1.7°F
(.94°C).
Also note that Table 25 on page 157 not only pro-
vides the formulas for the loading (ON POINT) and
unloading (OFF POINT) of the system, the “STEP” is
also shown in the table. The “STEP” is the increment
in the sequence of the capacity control scheme that can
be viewed under the OPER DATA key. See Display/
Print Keys on page 114 for specific information on
the OPER DATA key.
RETURN CHILLED LIQUID SYSTEM LEAD/
LAG AND COMPRESSOR SEQUENCING
A Lead/Lag option may be selected to help equalize
average run hours between systems with 2 refriger-
ant systems. This may be programmed under the OP-
TIONS key. Auto Lead/Lag of the 2 systems based on
average run hours of the compressors in each system.
Manual Lead/Lag selects specifically the sequence
which the microprocessor starts the systems.
The microprocessor will sequence compressors load
and unload systems according to Table 26 on page
158. The microprocessor will lead/lag compressors
within each circuit to maximize individual compres-
sor run time for the purpose of lubrication. It will also
prevent the same compressor from starting 2 times in
a row. The microprocessor will not attempt to equalize
run time on individual compressors within a system.
Each compressor in a system will be assigned an ar-
bitrary number 1, or 2. The non-running compressor
within a system with the lowest priority number will
always be the next compressor to start. The running
compressor with priority number 1 will always be the
next compressor to shut OFF. Whenever a compressor
is shut OFF, the priority numbers of all compressors
in each system will be decreased by 1 with the wrap
around. This control scheme ensures the same com-
pressor does not repeatedly cycle ON and OFF.
ANTI-RECYCLE TIMER
The programmable anti-recycle timer ensures that sys-
tems do not cycle. This timer is programmable under
the PROGRAM key between 300 and 600 s. When-
ever possible, to reduce cycling and motor heating, the
anti-recycle timer should be adjusted to 600 seconds.
The programmable anti-recycle timer starts the timer
when the first compressor in a system starts. The timer
begins to count down. If all of the compressors in a
circuit cycle OFF, a compressor within the circuit will
not be permitted to start until the anti-recycle timer has
timed out. If the lead system has run for less than 5
min, 3 times in a row, the anti-recycle timer will be
extended to 10 min.
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